Positron emission tomography (PET) scanners are utilized to generate images of, for example, portions of a patient's body. Positron attenuation data and/or annihilation events are utilized in generating such images. Positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. The radionuclides are employed as radioactive tracers called "radiopharmaceuticals" by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are injected into the patient and become involved in such processes as blood flow, fatty acids, glucose metabolism, and synthesis.
Positrons are emitted as the radionuclides decay. The positrons travel a very short distance before encountering an electron, and when that occurs, the positron is annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are directed in nearly opposite directions.
For detecting such events, the PET scanner has a ring of detectors that encircle the patient. The detectors comprise crystals, referred to as scintillators, to convert the energy of each 511 KeV photon into a flash of light that is sensed by a photomultiplier tube. Coincidence detection circuits connect to the detectors and record only those photons that are detected simultaneously by two detectors located on opposite sides of the patient The number of such simultaneous events indicates the number of positron annihilations that occurred along a line joining the two opposing detectors.
During a scan, hundreds of million of events are detected and recorded to indicate the number of annihilation events along lines joining pairs of detectors in the ring. The collected data is used to reconstruct an image. Further details regard PET scanners are set forth in U.S. Pat. Nos. 5,378,893, 5,272,343, and 5,241,181, all of which are assigned to the present assignee.
With respect to data collected during a scan, such data typically is normalized prior to using such data to reconstruct an image. Known normalization methods for three-dimensional imaging, however, are based on coincidence sensitivity for two-dimensional imaging. In accordance with these known methods, coincidence sensitivity is the product of the single-crystal efficiencies of the two detectors forming the coincidence and a "geometric factor", which in two-dimensional imaging is assumed to be a function of line of response (LOR) radius only and not a function of LOR angle. To extend the method to three-dimensional imaging, it is known to use the square root of the product of the two single-ring geometric factors for the geometric factor of a cross-plane LOR.
These previous methods have several drawbacks which make them inappropriate for three-dimensional imaging on some scanners. For example, with two-dimensional scanning, septa (or detector shields) are utilized so that each detector only detects events on a specific plane. With known three-dimensional methods, the septa are removed. The sensitivity of each crystal with the septa out, however, is assumed to equal the sensitivity with septa in. In an ideal scanner, this assumption may be true since the septa and crystals are exactly in their desired locations. As crystals get smaller and septa thinner, however, minor positioning errors of 1 mm or less become a significant departure from the ideal case. In real applications, such positioning errors are likely to occur.
In addition, a rod source may be used for acquisition of two-dimensional normalization data. Particularly, the rod source has a generally known rate of radionuclide decay and is placed within the ring detector. Axial non-uniformity of the rod source makes it impossible to measure relative slice sensitivities. Since the two-dimensional normalization procedure is concerned only with sensitivity variations within the slice, such a limited sensitivity is acceptable for two-dimensional imaging. For three-dimensional imaging, however, relative slice sensitivities are important.
Further, although the geometric factor in two-dimensional imaging is assumed to be a function of the LOR radius only, in three-dimensional imaging, the geometric factor is not a simple function of LOR radius. Other effects may introduce a strong angular effect with some period greater than one row. Manufacturing variations in the scanner ring, such as variations in module-to-module spacing in the detector assembly, may also produce additional variations in system sensitivity which must be taken into account by the three-dimensional geometric factor.
Accordingly, it is desirable to provide a data normalization array for three-dimensional imaging which takes into account the fact that the septa and crystals may not be exactly located in their desired position and compensates for sensitivity variations within a slice as well as for relative slice sensitivities. It also is desirable to provide such an array which takes into account effects such as variations in module-to-module spacing.